It's the sequence of odd numbers with alternating signs.
The sequence of positive odd integers 1,3,5,7,9,... has general
term 2n-1 (one less than the general term for the even integers, 2n)
To cause the signs to alternate, beginning with the first term being
positive, we must multiply by the factor (-1)n+1. So the
general term is
tn = (-1)n+1(2n-1)
Edwin
